Method of manufacturing aluminum-coated ferrous base articles

ABSTRACT

A method of coating a ferrous base metal with aluminum including cleaning the surface of the ferrous article by means of an aqueous solution of orthophosphoric acid having a concentration of about 5 to 85 percent. Subsequently, drying the surface to evaporate the water from said solution. Applying molten aluminum to the ferrous surface to establish a uniformly bonded aluminum coating by means of a layer of ferrous-aluminum intermetallic compound. The molten aluminum application being effected without any additional application of fluxing material to the ferrous surface after the cleaning operation.

United States Patent 1,694,820 12/1928 Harris Inventor William C. Patterson Turtle Creek, Pa.

App]. No. 851,474

Filed Aug. 19, 1969 Patented Jan. 4, 1972 Assignee Aluminum Company of America Pittsburgh, Pa.

METHOD OF MANUFACTURING ALUMINUM- COATED FERROUS BASE ARTICLES 11 Claims, No Drawings US. Cl 148/615 R,

117/51, 117/1 14 C, 134/41 Int. Cl C231 7/10 Field ofSearch l48/6.15,

References Cited UNITED STATES PATENTS 2,310,451 2/ l 943 Marshall 148/615 2,738,289 3/1956 Hodge 117/51 2,818,360 12/1957 Porter 148/12.1 FOREIGN PATENTS 161,605 3/1964 U.S.S.R. 117/114 Primary Examiner-Ralph S. Kendall Attorney-Amold B. Silverman METHOD OF MANUFACTURING ALUMINUM-COATED FERROUS BASE ARTICLES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to a method for coating a ferrous article with aluminum by hot dipping. More specifically, it relates to such a method wherein use of a specific cleaning material eliminates the need for subsequent fluxing.

2. Description of the Prior Art It has been known that for numerous ferrous products desirable improvements may be obtained by applying continuous coatings of aluminum. Not only do such coatings provide substantial resistance to oxidation, acid attack and other corrosion of ferrous products, but also increased product electrical conductivity, reflectivity and attractiveness may be provided. Numerous methods of applying an aluminum coating to a ferrous product have been previously known. Among the known systems are electroplating, spraying, cladding, and casting aluminum around steel.

One of the major problems of such known systems has been their inadequacy with respect to providing a uniform coating which is securely bonded to ferrous articles of irregular shape. One method of avoiding the undesirable characteristics of such systems has been through the use of hot dip aluminizing." Such a process involves coating ferrous articles with aluminum by immersing a suitably prepared ferrous product into a vessel containing molten aluminum.

In general, known systems require preliminary cleaning and subsequent pickling of the ferrous article. This is followed by fluxing, rinsing, drying and immersion in molten aluminum. The initial cleansing of the ferrous article may be effected by the rather cumbersome and expensive process of grit blasting. In addition, grit blasting requires the use of special, limited purpose equipment. It has been conventional to employ two preliminary cleaning steps in addition to or in lieu of the grit blasting. It has been known to first remove grease and oils which may be adhered to the surface of the ferrous product. This might be accomplished by any conventional means such as the use of an organic solvent, an alkaline cleaner or by vapor degreasing. Subsequent to degreasing, it has been customary to treat the ferrous surface by acid pickling. This is generally accomplished by employing a strong acid such as hydrochloric or sulfuric acid in order to remove rust and scale from the ferrous surface.

After the pickling operation, the ferrous surface is then conventionally rinsed with water and dried. A suitable flux is then applied to the ferrous surface in order to prevent the formation of oxidation products on the surface and provide a clean surface which may be subsequently uniformly wetted by the molten aluminum. The use of molten salt in dips as fluxes has been known. Acids and alkaline materials have also been used for this purpose.

U.S. Pat. No. 2,738,289 discloses the use of an alkaline salt of an oxyacid of phosphorous with an alkaline metal as a fluxing material. Molten salt baths are also disclosed in U.S. Pat. No. 2,544,671 and 2,569,097. The use of phosphoric acid in very small concentrations has been disclosed in U.S. application Ser. No. 758,709, filed Sept. 10, l968, now abandoned.

After fluxing, the ferrous article is generally dried and subsequently immersed in the vessel of molten aluminum. Upon removal from the vessel the excess aluminum adhering to the ferrous article is then removed either manually or mechanically as by vibration or centrifugal action.

One of the major problems encountered in connection with the known systems is the amount of work, material and equipment which are directed toward the preliminary process steps involved in preparing the ferrous article for hot dipping. The problem is accentuated by reason of the important role which economic considerations play with respect to commercial feasibility of such conversion processes.

SUMMARY OF THE INVENTION The method of this invention has solved the above-mentioned problems encountered in hot dip aluminizing of ferrous base metals. It employs a pickling material which when used in the manner contemplated by this invention eliminates the need for subsequent use of a fluxing material. The method of this invention provides for the use of an aqueous'solution of orthophosphoric acid in order to pickle the surface of the ferrous article. The article is subsequently dried in order to evaporate the water from the ferrous surface and is immersedin the molten aluminum without the need forsubsequent fluxing. Upon immersion into the molten aluminum bath, the aluminum unifonnly reacts with the ferrous surface to establish a bond by formation of a layer of ferrous-aluminum intermetallic compound. The orthophosphoric acid when applied as provided herein not only effectively cleanses the ferrous surface to remove rust and scale, but also provides a phosphate coating which permits subsequent formation of the intermetallic component.

In a preferred form of the invention, the orthophosphoric acid solution has a concentration of about 5 to l5 percent and cleansing of the ferrous surface is effected over a period of about 4 to 15 minutes. In this form of the invention no rinsing is required intermediate the cleaning and drying operations.

It is an object of this invention to provide a method for aluminizing ferrous articles wherein a specific cleaning operation effectively cleanses the ferrous surface and also eliminates the need for subsequent fluxing of the article prior to immersion in molten aluminum.

It is another object of this invention to provide such a system which will yield a smooth unifonnly bonded aluminum coating on the ferrous article secured thereto by means of a layer of ferrous-aluminum intermetallic compound.

It is another object of this invention to provide a hot dip aluminizing process which may be economically practiced by reducing the amount of required pretreatment materials and the amount of required process steps, while providing a superior end product.

These and other objects will be more fully understood and appreciated from the following detailed description which is exemplary and explanatory of the invention and the best mode of practicing the invention now contemplated by applicant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of this invention contemplates aluminizing a ferrous base article to provide a smooth uniformly adherent coating of aluminum. An initial cleaning operation is performed and without any subsequent application of flux the ferrous article may be immersed in molten aluminum to provide the coating. The cleaning operation may be effected by means of solutions or vapors customarily used for removal of oil, grease and the like followed by an aqueous solution of orthophosphoric acid. Intermediate the cleaning operation and immersion of the article in molten aluminum, the ferrous article is dried in order to evaporate the water from the aqueous solution. This prevents damaging explosions caused by rapid, substantial transformation of water to vapor should the former contact molten aluminum. Also, in some instances it may be desirable to rinse the surface of the ferrous article prior to drying.

The term aluminum as used herein shall refer to aluminum of any grade and aluminum base alloys wherein aluminum consists of not less than percent by weight of the total composition.

In some instances, the pickling operation of this invention may be the only required cleaning operation. In this sense cleaning is used to refer to removal of rust, scale and similar corrosion products disposed on the surface of the ferrous article. As has been noted above, ferrous articles will frequently have a quantity of adherent grease or oil on the surface. It is, of course, entirely consistent with this invention to degrease the surface by any conventional means prior to the pickling or cleaning operation of this invention. The use of the term cleaning" herein shall refer to pickling but shall not preclude the use of a prior degreasing precleaning step before pickling. The cleaning or pickling operation contemplated by this invention employs an aqueous solution of orthophosphoric acid having an acid concentration of about to 85 percent. The cleaning is accomplished within a period of about 4 to minutes. The temperature of the acid solution may range from ambient room temperatures (about 70 F.) up to about 200 F. These three variables each have an influence on the rate of cleaning. Cleaning action may be accelerated by increasing any of these variables, i.e., acid concentration, period of immersion or temperature of the aqueous acid solution. The ranges provided by this invention effectively cleanse the ferrous surface and yet eliminate subsequent fluxing and in some instances eliminate rinsing. As will be described in detail below, some ranges are preferred for increased effectiveness.

It is critical that the orthophosphoric acid concentration be a minimum of 5 percent, as below that concentration insufficient cleaning action is obtained, unless application is effected over an unduly long, commercially impractical period of time. While the operable range of concentrations extends upwardly to about 85 percent acid concentration, no meaningful improvement in the effectiveness of the process is noted above about 15 percent acid concentration. For economic reasons related to the cost of acid materials, this range is a preferred range. It should be noted, however, that while an increase in acid concentration does reduce the required treatment time, it may become commercially advantageous to use a higher range of acid concentration and take advantage of the opportunity to increase the production rate by reducing the period of immersion in the aqueous acid solution.

With respect to ferrous articles generally, the immersion period preferred is about 4 to 15 minutes. The period of immersion during the cleaning operation of cast iron materials is preferably about 5 to 12 minutes, with a period of 5 to 8 minutes being preferred for orthophosphoric acid concentrations of about 50 to 85 percent.

While it has been found that a good smooth aluminum coating uniformly bonded to the ferrous surface may be effected throughout the acid concentration ranges of 5 to 85 percent, an added benefit may be obtained in the lower portions of the concentration ranges. in these ranges, no water rinsing is required intermediate the cleaning operation and the drying operation which precedes application of the molten aluminum to the ferrous article. With respect to cast iron, it has been found that no intermediate rinse is required for aqueous solutions having acid concentrations ranging from 5 percent to about 49 percent. In the range of concentrations of 50 to 85 percent some impairment in adhesion properties is produced where an intermediate rinse is not employed. With respect to steel products, it has been noted that good coatings may be produced in the range of acid concentration of about 5 to percent without requiring intermediate rinsing. In the range of concentrations of from about 21 to 85 percent some coating roughness results where the intermediate rinse is not employed. In these upper ranges it is apparently desirable to rinse, preferably with water, which may be tapwater or deionized water, after cleaning if a good smooth continuously bonded coating is to be provided.

The invention, therefore, provides a lower preferred range of orthophosphoric acid concentration which is economically advantageous in terms of the cost of acid materials, a somewhat enlarged intermediate range which might provide economic advantages in terms of reduced immersion time and a broad range which is operable to produce a good smooth, uniformly bonded aluminum coating on a ferrous article.

The cleaning or pickling operation of this invention results in some etching of the ferrous article s surface to remove oxidation products therefrom and provides a protective phosphate coating or film over the cleansed surface. The phosphate coating produced on the ferrous surface is preferably of about 0.4 to 1.3 mg./in. This quantity is effective to prevent any detrimental oxidation prior to immersion in the molten aluminum and facilitates formation of the ferrous aluminum intermetallic compound during the hot dip operation.

Upon immersion of the ferrous article into the molten aluminum bath, the aluminum diffuses into the ferrous base metal and forms a layer of ferrous aluminum intermetallic compound. It is this compound which serves to effectively secure the aluminum material to the ferrous base substrate. This intermetallic ferrous aluminum compound layer can vary in thickness from about 0.0005 to 0.015 inch. In some instances it is preferable to provide an aluminum coating which has an inner layer of ferrous aluminum compound which, in thickness, is about 15 to 50 percent of the thickness of the aluminum coating and an outer layer of aluminum which, in thickness, is about 50 to percent of the thickness of the entire coating. Under conditions where exposure to high temperatures is involved, it might be desirable to provide an aluminum layer which is substantially completely formed of the intermetallic compound and has only a thin overlying film of aluminum.

With respect to the matter of cleaning, precleaning of the ferrous products is not required to produce the coating properties desired unless grease and oil contamination is substantial. The application of the aqueous orthophosphoric acid solution is sufficient to properly cleanse the ferrous surface. In instances where it is desired, and in instances where higher acid concentrations are employed, the cleaning operation may be followed by a rinse to remove the freed dirt particles. The article is then dried at either ambient room temperature or elevated temperature. If desired, a current of moving air may be employed to dry the surface. The ferrous article is then immersed in the molten aluminum bath in conventional fashion.- The bath temperature is generally about l,300 to 1,350 F. and immersion is effected over a period of adequate duration to bring the article to the operating temperature range. This depends on furnace capacity, article weight and article configuration. An aluminum coating of 0.001 to 0.005 inch thick is preferably produced.

If desired, in the preparation of the aqueous orthophosphoric acid solution quantities of polyphosphoric acid may also be included. To the extent to which the polyphosphoric acid would revert to orthophosphoric acid in the aqueous solution,

Example 1 Two groups of cast-iron articles were immersed in 5 and [0 percent aqueous solutions of orthophosphoric acids, respectively, at ambient room temperatures. None of the articles had been precleaned or degreased. One group of articles was maintained in the 5 percent acid solution for about 12 minutes and the other was maintained in the l0 percent acid solution for about 10 minutes. The specimens were then removed from the acid solutions and articles from each group were rinsed with water (group A), while the remaining articles were not (group B). All of the articles were then dried at about 70 F. for 10 minutes to evaporate the water and were subsequently immersed in molten aluminum at l,325 F. for 3 minutes. All of the articles from both groups A and B were observed to have a good smooth, uniformly bonded coating of aluminum with the desired appearance.

Example 2 The tests of example 1 were repeated except for minor degreasing of the cast-iron specimens and the use of 15, 20 and 30 percent orthophosphoric acid solutions. The articles were inserted into the respective solutions for periods of 8 minutes. The specimens of both groups A and B exhibited good smooth, uniformly bonded coatings with the desired surface characteristics.

Example 3 Three groups of cast-iron specimens (groups C, D and E) were subjected to degreasing and were introduced into 50, 65 and 85 percent orthophosphoric acid solutions, respectively, for periods of 5, 6 and 8 minutes, respectively. Some specimens from each group C, D and E were then rinsed (group X), while others (group Y) were not. The specimens were then dried and immersed in molten aluminum. The specimens of group X exhibited the same good smooth, uniform coating as in the previous examples, while the specimens of group Y exhibited some roughness characteristic of a lack of adhesion either in isolated areas or in a number of regions.

Example 4 A group of mild steel specimens (carbon content between 0.15 to 0.25 percent) were cleaned by immersion in 10, 21, 42 and 85 percent solutions of orthophosphoric acid, respectively, at 140 F. for periods of 9, 8, 4 and minutes, respectively. None of the articles were precleaned or degreased. A first group (group M) from each solution was subsequently rinsed with water, while a second group (group N) was not. The specimens were then dried and dipped into molten aluminum at 1,325 E for 3 minutes. All of the group M specimens were found to have a good smooth, uniformly bonded aluminum coating with the desired surface characteristics, as were the group N specimens which had been immersed in the 10 percent acid solution. The remaining group N specimens were observed to have a generally good well-adhered coating with some minor roughness.

These examples clearly establish the effectiveness of applicant's process in producing a good smooth aluminum coating on a ferrous base metal article. The coating is uniform in thickness and possesses strong adherent properties with respect to the ferrous substrate. No precleaning operation was necessary in this instance and also the application of an independent fluxing agent after the cleaning operation is eliminated. In addition, in the economically advantageous preferred lower ranges the use of a rinse after cleaning and prior to drying is eliminated.

All of these economic advantages which lead to the production of a good commercial product are performed by the process of this invention without the need for capital investment in expensive limited use equipment. Also, as the process is simplified, reduced training of workmen and less effort on the part of workmen are facilitated.

While throughout the description reference has been made frequently to immersion of the ferrous article in the aqueous orthophosphoric acid solution, it will be appreciated that application of the acid solution may be readily effected by other conventional means such as spraying, mist chambers, rollingon or wiping (swabbing), for example.

It will be appreciated that the primary purpose of the process of this invention is the improved pretreatment of ferrous surfaces for receipt of aluminum coatings. This coating may be advantageously applied by hot dip aluminizing, but other means of application of the molten aluminum to the ferrous article may in some instances prove advantageous, be the process a batch or continuous process. As has been noted above hot dip aluminizing is particularly beneficial where the ferrous material is a fabricated or semifabricated article of irregular configuration.

While particular embodiments of the invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details may be made without departing from the invention as defined in the appended claims.

I claim:

1. A method of coating a ferrous base metal with aluminum comprising,

providing a ferrous base metal article,

cleaning the surface of said ferrous article by means of an aqueous solution of orthophosphoric acid having an orthophosphoric acid concentration of about 5 to percent,

drying said surface to evaporate the water of said solution,

applying molten aluminum to said ferrous surface to establish a uniformly bonded aluminum coating by means of a layer of ferrous-aluminum intermetallic compound,

applying said molten aluminum by immersing said ferrous article into a molten aluminum bath, and

said molten aluminum application being effected without any additional application of fluxing material to said ferrous surface after said cleaning operation.

2. The method of claim 1 including,

said cleaning of said ferrous surface by said aqueous orthophosphoric acid solution is accomplished within a period of about 4 to 15 minutes, and

said aqueous solution has an orthophosphoric acid concentration of about 5 to 15 percent.

3. The method of claim 1 including,

said ferrous base metal article is composed of cast iron,

said aqueous solution has an orthophosphoric acid concentration of about 50 to 85 percent,

said cleaning operation is accomplished within about 5 to 8 minutes, and

after said cleaning operation but prior to said drying operation rinsing the surface of said ferrous base metal with water.

4. The method of claim 1 including,

said ferrous base metal article is composed of steel,

said aqueous solution has an orthophosphoric acid concentration of about 20 to 85 percent,

said cleaning operation is accomplished within about 8 to 15 minutes, and

after said cleaning operation but prior to said drying operation rinsing the surface of said ferrous base metal with water.

5. The method of claim 2 including,

said ferrous base material is composed of cast iron,

said aqueous solution has an orthophosphoric acid concentration of about 5 to 10 percent, and

prior to said cleaning operation, no precleaning operation is performed.

6. The method of claim 2 including,

prior to said cleaning operation, no precleaning operation is performed.

7. The method of claim 2 including,

said ferrous surface after said drying operation has a phosphate coating of about 0.4 to 1.3 mg./in.

said molten aluminum is applied to said ferrous surface by immersing said ferrous base metal therein, and

said phosphate coating is sufficiently thin to permit diffusion of said molten aluminum into said ferrous base metal with resultant fonnation of a layer of a ferrous-aluminum intermetallic compound overlying the ferrous base metal.

8. The method of claim 7 wherein,

said intermetallic ferrous-aluminum compound has a thickness of about 0.001 to 0.015 inch.

9. The method of claim 8 wherein,

said cleaning is effected at a temperature of about 70 to said molten aluminum into which said ferrous base metal is immersed is at a temperature of about l,300 to 1,350 F., and

said immersion is effected for about 1 to 5 minutes.

10. The method of claim 9 including,

producing said aluminum coating substantially completely in the form of said ferrous-aluminum intermetallic compound with a thin overlying aluminum layer.

11. The method of claim 10 including,

said aluminum coating having an inner layer of ferrous-aluminum intermetallic compound about 15 to 50 percent of the thickness of said aluminum coating and an outer layer of aluminum about 50 to 85 percent of the thickness of said aluminum coating.

i i i i i 

2. The method of claim 1 including, said cleaning of said ferrous surface by said aqueous orthophosphoric acid solution is accomplished within a period of about 4 to 15 minutes, and said aqueous solution has an orthophosphoric acid concentration of about 5 to 15 percent.
 3. The method of claim 1 including, said ferrous base metal article is composed of cast iron, said aqueous solution has an orthophosphoric acid concentration of about 50 to 85 percent, said cleaning operation is accomplished within about 5 to 8 minutes, and after said cleaning operation but prior to said drying operation rinsing the surface of said ferrous base metal with water.
 4. The method of claim 1 including, said ferrous base metal article is composed of steel, said aqueous solution has an orthophosphoric acid concentration of about 20 to 85 percent, said cleaning operation is accomplished within about 8 to 15 minutes, and after said cleaning operation but prior to said drying operation rinsing the surface of said ferrous base metal with water.
 5. The method of claim 2 including, said ferrous base material is composed of cast iron, said aqueous solution has an orthophosphoric acid concentration of about 5 to 10 percent, and prior to said cleaning operation, no precleaning operatiOn is performed.
 6. The method of claim 2 including, prior to said cleaning operation, no precleaning operation is performed.
 7. The method of claim 2 including, said ferrous surface after said drying operation has a phosphate coating of about 0.4 to 1.3 mg./in.2, said molten aluminum is applied to said ferrous surface by immersing said ferrous base metal therein, and said phosphate coating is sufficiently thin to permit diffusion of said molten aluminum into said ferrous base metal with resultant formation of a layer of a ferrous-aluminum intermetallic compound overlying the ferrous base metal.
 8. The method of claim 7 wherein, said intermetallic ferrous-aluminum compound has a thickness of about 0.001 to 0.015 inch.
 9. The method of claim 8 wherein, said cleaning is effected at a temperature of about 70* to 200* F., said molten aluminum into which said ferrous base metal is immersed is at a temperature of about 1,300* to 1,350* F., and said immersion is effected for about 1 to 5 minutes.
 10. The method of claim 9 including, producing said aluminum coating substantially completely in the form of said ferrous-aluminum intermetallic compound with a thin overlying aluminum layer.
 11. The method of claim 10 including, said aluminum coating having an inner layer of ferrous-aluminum intermetallic compound about 15 to 50 percent of the thickness of said aluminum coating and an outer layer of aluminum about 50 to 85 percent of the thickness of said aluminum coating. 